In the iron and steel industry, it is necessary to treat the ferrous base metals while in the molten state with a desulfurizing agent to reduce the sulphur content of the metal product.
Magnesium metal is a powerful deoxidizer and desulfurizer. However, this metal boils at relatively low temperatures and therefore, the sudden increase in volume which is produced when the magnesium metal is added to the molten iron, may result in violent reactions as the magnesium is vaporized.
Various methods have been used to reduce this violent activity by slowly introducing the magnesium into molten ferrous metal under rigidly controlled systems. One of these methods for reducing the violence using magnesium metal is to impregnate porous bodies with the magensium metal and to plunge the infiltrated body into the molten ferrous metal. Under these conditions, the impregnated magnesium metal is released at a slow enough rate that the violence is held to minimum.
The scrap ferrous metal network described above has been impregnated with magnesium metal. This impregnated network is more fully described and claimed in our copending applications Ser. No. 454,951 filed Mar. 26, 1974 and Ser. No. 514,630 filed Oct. 15, 1974.
In some applications, however, magnesium metal is not effective. For instance in desulfurizing steel, it is very difficult to reach the desirably low sulfur levels with magnesium and other agents. Also in producing ductile iron, if impurities such as lead and tin are present, good nodularization of the graphite may not be obtained by the addition of magnesium alone. In these cases the problems can be overcome by employing rare earth metals instead of, or in addition to, magnesium metal.
Rare earths are reactive elements however, which oxidize rapidly, particularly in the molten state. Since they are very expensive, high efficiency is essential. Rare earths are currently added to ferrous melts in several ways. The rare earths may be placed at the bottom of a ladle just before the melt is poured in or may be attached as "donuts" on a rod and plunged in the melt. Rare earths can also be added as silicides but the concurrent addition of Si may introduce problems. Good efficiency can be achieved by using relatively dilute alloys such as Fe-30 Si-2Mg-2RE, but these alloys are expensive and the introduction of silicon may pose problems.
In contrast to the prior art, the instant invention provides means to introduce rare earth metals into molten iron or steel in a porous ferrous briquette. This is accomplished by impregnating the compressed ferrous metal network briquettes of the copending applications cited above with rare earth metal instead of with magnesium metal. These briquettes impregnated with rare earth metals are added in such a way that they remain below the melt surface until they have substantially dissolved. The gradual dissolution of the briquettes introduce rare earths into the melt at a controlled rate so that they can be efficiently utilized. The scrap portion of the briquette is ferrous metal (usually mild steel) and introduces no undesirable impurities into the melt.
The metal network, prepared in the copending applications which serve as the carrier for the rare earth metals, comprises a mass of scrap ferrous metal pieces compressed together in random orientation, forming a network of interlocking pieces, said mass having a density of 1.2 to 6.3 g/cc, a porosity of 20% to 85%, and a short transverse tensile strength of at least 2.0 psi, preferably at least 2.5 psi.
This mass of porous ferrous metal network contains a labyrinth of interstices in the voids between the compressed interlocking metal pieces. These interstices may be filled with rare earth metal by immersing the ferrous metal network in molten rare earth metal and solidifying the molten metal impregnated throughout the interstices of the porous metal network. The amount of rare earth metal which may be impregnated into the ferrous metal mass may be from 15% to 80% by weight of the impregnated body. This product is useful for treating ferrous melts, for example, for desulfurizing steel and also for producing nodular iron. Normally about 1/2 to 2 pounds of rare earth metal are used to desulfurize 1 ton of molten steel. The same amounts of rare earth metals also are used to treat one ton of nodular iron. Usually iron is or steel is treated first with a less expensive agent, e.g. magnesium metal, before rare earth metals are employed.